JPH071808Y2 - Semiconductor laser drive - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION <Industrial Application Field> The present invention relates to a semiconductor laser driving device for driving a semiconductor laser in which a semiconductor laser chip and a light receiving element for monitoring are built in the same package. In particular, the present invention relates to a semiconductor laser driving device for pulse driving, which includes a circuit for stabilizing the optical output of a semiconductor laser based on a light receiving signal of the light receiving element.

<Prior Art> As shown in FIG. 4, a conventional semiconductor laser driving device includes a laser diode LD, a monitor photodiode PD built in the same package 4 as the laser diode LD, and the photodiode PD. And a transistor control section 3 for controlling the drive current of the laser diode LD by amplifying the photocurrent of PD with the two transistors 1 and 2, and the transistor control section 3 has two biases. resistors R _1, R _2, circuit elements such as a volume resistance V _R for the output light amount adjustment of the laser diode LD is provided as appropriate.

In the device example of the same figure, when a pulse voltage as shown in Fig. 3 (1) is applied as the power supply voltage V _CC , the PNP transistor 2 is turned on at the rising edge of this pulse, and the drive current to the laser diode LD is increased. Flow to generate a light output. When this light output is monitored by the photodiode PD, a photocurrent I _L is generated and when the value of I _L × (R ₁ + V _R ) exceeds the base-emitter voltage V _BE of the NPN transistor 1. , The transistor 1 is turned on. As a result, a current flows through the bias resistor R ₂ , the base potential of the transistor 2 is raised, and the transistor 2 is cut off, so that the drive current of the laser diode LD is reduced. Therefore, in the steady state, the laser diode LD of this device example is a light that becomes I _L × (R ₁ + V _R ) ≈V _CC -V _F (where V _F is the forward voltage drop of the laser diode LD). It will be fixed to the output.

<Problems to be Solved by the Invention> However, in the above device example, the transistor 1 is turned on after the transistor 2 is turned on.
The transistor 2 is saturated in a very short time until the base potential of the transistor 2 is raised, and the cutoff cannot be performed immediately even if the base potential rises. As a result, the drive current waveform of the laser diode LD becomes a waveform including the overshoot portion 4 as shown in FIG. 3 (2).
However, there is a problem that an excessive current flows for a moment to accelerate its deterioration and shorten the life of the laser diode LD.

The present invention has been made in view of the above problems, and has a circuit configuration for preventing the occurrence of the overshoot portion in the drive current waveform of the semiconductor laser, thereby extending the life and reliability of the semiconductor laser. It is an object of the present invention to provide a novel semiconductor laser drive device that realizes the improvement.

<Means for Solving Problems> In order to achieve the above-mentioned object, in the present invention, a semiconductor laser and an optical output of the semiconductor laser incorporated in the same package as the semiconductor laser are received and variations in the optical output are detected. In a semiconductor laser drive device comprising a light receiving element for, and a control section for controlling the operating current of the semiconductor laser according to a change in the output of the light receiving element to hold the optical output of the semiconductor laser constant, the control section is A PNP-type first transistor that connects a semiconductor laser to the emitter side to control the operating current of the semiconductor laser, and a second transistor that applies a signal corresponding to an output change of the light-receiving element to the base of the first transistor
And a resistor connected between the emitter of the first transistor and the semiconductor laser so as to always operate the first transistor in the active region.

<Operation> In the control unit, the PNP-type first transistor operates as a linear amplifier body which is always in an active state. This eliminates the time delay of the negative feedback from the light receiving element, and the rise of the drive current waveform of the semiconductor laser. Will be faster. Therefore, an overshoot portion is prevented from being generated in the drive current waveform of the semiconductor laser, an excessive current is prevented from flowing to the semiconductor laser, the life of the semiconductor laser is extended, and the reliability is improved.

<Embodiment> FIG. 1 shows an example of a circuit configuration of a semiconductor laser driving device according to an embodiment of the present invention. A laser diode LD and the same package 4 as the laser diode LD are shown.
The photo diode PD built in the photo diode PD and the photo current of the photo diode PD are amplified by the NPN type transistor 1 and the PNP type transistor 2 having a large power amplification factor h _FE (about 400 times) and the laser diode LD The transistor control unit 3 controls the drive current. The photodiode PD is for receiving a part of the light output of the laser diode LD and detecting a change in the light output thereof, and one of the transistors 1 forming the transistor control unit 3 on the anode side of the photodiode PD. The base is connected.
The emitter of the transistor 1 is connected to the base of the other transistor 2 forming the transistor control unit 3, and the emitter of the transistor 2 is connected to the cathode of the laser diode LD via the emitter resistor R _E. is there. The emitter resistance R _E is for keeping the transistor 2 in a non-saturated state and always operating in the active region.

In the figure, R ₁ and R ₂ are bias resistors, and V _R is a volume resistor for adjusting the output light amount of the laser diode LD. Further, V _CC indicates a power supply voltage and GND indicates a ground, and a pulse voltage waveform as shown in FIG. 3 (1) is given as the power supply voltage V _CC .

In the above embodiment, the insertion position of the emitter resistor R _E is not limited to the position between the laser diode LD and the transistor 2,
For example, it may be between the power supply voltage V _CC and the laser diode LD. Also laser diode LD and photodiode
Even in the embodiment shown in FIG. 2 in which the connection method with the PD is different, the emitter resistor R _{E is connected} to the emitter side of the transistor 2.
Of course, it is connected.

Next, the operation when the example of the apparatus shown in FIG.

In the illustrated example, the power supply voltage V _CC is now shown in FIG.
Assuming that the pulse voltage shown in (1) is applied, the transistor 2 is turned on at the rising edge of this pulse, a drive current flows through the laser diode LD, and an optical output is generated. When this light output is received by the photodiode PD, a photocurrent is generated and flows through the bias resistor R ₁ and the volume resistor V _R to turn on the transistor 1. As a result, a current flows through the bias resistor R ₂ to raise the base potential of the transistor 2 and the transistor 2 is cut off, so that the drive current of the laser diode LD decreases.

In this case, there is a time delay until the transistor 1 is turned on and the base potential of the transistor 2 rises, but the transistor 2 does not become saturated during that period because the emitter resistance R _E exists. , Always operates as an active linear amplifier. Therefore, as the base potential of the transistor 2 rises, the transistor 2 is cut off immediately, and the drive current of the laser diode LD decreases. As a result, the drive current waveform of the laser diode LD rises quickly and does not include an overshoot portion, as shown in FIG.

Next, in the case where the laser diode LD is set under a predetermined light output state, assuming that the light output of the laser diode LD is lowered due to temperature change, in this case, the photodiode PD is The amount of light received decreases and the photocurrent decreases. This lowers the base potential of the transistor 1 and
The current flowing therethrough decreases and the emitter potential of the transistor 1 decreases. On the other hand, in the transistor 2, the potential difference between the base and the emitter becomes large, so the laser diode LD
To act on increasing the drive current, and the optical output of the laser diode LD is increased.

On the other hand, if the optical output of the laser diode LD increases due to temperature changes, the photodiode PD will
The amount of light received from the LD increases and the photocurrent increases. As a result, the base potential of the transistor 1 rises, the current flowing through the transistor 1 increases, and the emitter potential of the transistor 1 also rises. On the other hand, since the transistor 2 has a small potential difference between the base and the emitter, the transistor 2 acts on the laser diode LD in the direction of reducing the operating current, and the optical output of the laser diode LD is reduced.

<Effects of the Invention> As described above, the present invention is the first PNP type that controls the operating current of the semiconductor laser by connecting the semiconductor laser to the emitter side of the control unit for keeping the optical output of the semiconductor laser constant. Second transistor and a second transistor for applying a signal corresponding to a change in the output of the light receiving element to the base of the first transistor.
And a resistor connected between the emitter of the first transistor and the semiconductor laser so that the first transistor is always operated in the active region by the resistor. The purpose of the invention is to prevent overshoot from occurring in the laser drive current waveform, prevent an excessive current from flowing to the semiconductor laser, and realize a longer life and improved reliability of the semiconductor laser. The remarkable effect achieved is achieved.

[Brief description of drawings]

FIG. 1 is an electric circuit diagram showing a circuit configuration example of a semiconductor laser driving device according to an embodiment of the present invention, FIG. 2 is an electric circuit diagram showing another embodiment of the present invention, and FIG. 3 is a power supply voltage waveform. FIG. 4 is an explanatory view showing a comparative example of the drive current waveform of the laser diode with respect to FIG. 4, and FIG. 4 is an electric circuit diagram showing a conventional example. LD …… Laser diode PD …… Photodiode R _E …… Emitter resistance 1,2 …… Transistor 3 …… Transistor controller

Claims (3)

[Scope of utility model registration request] 1. A semiconductor laser, a light receiving element which is built in the same package as the semiconductor laser and which receives the optical output of the semiconductor laser and detects fluctuations in the optical output, and a semiconductor according to the output change of the light receiving element. In a semiconductor laser drive device including a control unit for controlling an operating current of a laser to maintain a constant optical output of the semiconductor laser, the control unit connects the semiconductor laser to an emitter side to control an operating current of the semiconductor laser. A PNP first transistor to be controlled; a second transistor which gives a signal according to an output change of the light receiving element to the base of the first transistor; an emitter of the first transistor and the semiconductor laser; A semiconductor laser driving device comprising: a resistor connected between the resistors for constantly operating the first transistor in an active region. 2. The semiconductor laser driving device according to claim 1, wherein the semiconductor laser is a laser diode. 3. The semiconductor laser driving device according to claim 1, wherein the light receiving element is a photodiode.